A PVA film is generally formed by a casting process making use of water as a solvent, or an extrusion process. The PVA film features that it is flexible and antistatic, and moreover has the best oxygen gas barrier property among general synthetic resin films in a dry state. Therefore, the use of the PVA film has heretofore been intended to develop to the field of packaging materials, for which good oxygen gas barrier property is required, making good use of this gas barrier property. However, the oxygen gas barrier property of the PVA film greatly depends on humidity and are hence impaired to a great extent due to moisture absorption under high-humidity conditions. Further, the PVA film is easy to dissolve in boiling water.
A PVA film has hitherto been used in the form of a laminate film of a multi-layer structure having at least two layers of the PVA film and another film when using it for a purpose in which practical oxygen gas barrier property is required, whereby the influence of humidity has been lessened as much as possible. In the method forming the laminate film, however, moisture resistance and water resistance are still insufficient. There are thus demands for improving the water resistance of the PVA itself and allowing the PVA film to satisfactorily keep the good oxygen gas barrier property even under high-humidity conditions.
In order to solve the above problems involved in the PVA film, various investigations such as, for example, those described below have heretofore been made.
(1) To impart water resistance by chemical modification of hydroxyl groups in PVA:
It has been known to convert hydroxyl groups in PVA into a form of acetal with aldehydes. However, this method has a disadvantage that although water insolubility can be imparted to PVA, the oxygen gas barrier property of the resulting films is markedly lowered if absorbing water.
In U.S. Pat. No. 2,169,250, it has been proposed to form films, fibers and the like from an aqueous mixture solution containing PVA and a polycarboxylic acid, and heat the formed products to react the hydroxyl groups in PVA with the polycarboxylic acid so as to form a crosslinked structure, whereby the formed products are made insoluble in water.
(2) To impart water resistance by heat treatment:
A PVA film tends to crystallize by heat treatment, so that its water resistance is improved. In addition, biaxial stretching of the PVA film can facilitate its orientation and crystallization, and moreover improve its mechanical properties. Therefore, it has been known to subject a PVA film to biaxial stretching and heat treatment, thereby improving its water or moisture resistance. However, this method has a disadvantage that although water insolubility can be imparted to the PVA film, the oxygen gas barrier property of the film is markedly lowered if absorbing water. In particular, the film undergoes deformation and changes in physical properties due to moisture absorption under high-humidity conditions.
(3) To impart water resistance by polyvinylidene chloride latex coat:
It has been known to coat a PVA film with a polyvinylidene chloride latex so as to impart moisture resistance to the PVA film. However, chlorine gas attributable to chlorine in the polyvinylidene chloride generates upon its incineration in waste treatment, and this method hence involves a problem from the viewpoint of environment.
(4) To impart water resistance by copolymerization:
When a copolymer of ethylene and vinyl acetate is hydrolyzed, an ethylene-vinyl alcohol copolymer (EVOH) is obtained. An EVOH film is a film combining good oxygen gas barrier property which is a feature of PVA films with properties inherent in thermoplastic films. However, the oxygen gas barrier property of the EVOH film greatly depends on humidity. The improvement in prevention against the reduction of the oxygen gas barrier performance due to moisture absorption is not yet sufficiently made.
As described above, the conventional measures for imparting water resistance to PVA films are still insufficient from the viewpoint of keeping the excellent oxygen gas barrier performance of the PVA films under dry conditions even under conditions of high humidity or high temperature and humidity.
On the other hand, there have been proposed films and sheets making use of a mixture of PVA and polyacrylic acid (for example, Japanese Patent Publication Nos. 47743/1988, 14376/1990 and 27941/1990). These films and sheets all are water-soluble or water-absorbable and not films having good water resistance and oxygen gas barrier property.
Incidentally, the above-described U.S. Pat. No. 2,169,250 also discloses the use of polymethacrylic acid or polyacrylic acid as the polycarboxylic acid for reacting with PVA. As a specific example thereof, it is described to polymerize a methacrylic acid monomer in a solution of PVA in water, cast the resulting reaction mixture on a support, evaporate the water, and then heat the dry film for 5 minutes at 140.degree. C., thereby reacting PVA with polymethacrylic acid to obtain a water-insoluble film (Example I). According to the results of an investigation by the present inventors, however, any film exhibiting excellent oxygen gas barrier property under high-humidity conditions can not be obtained by this heat treatment conditions. Besides, even if other specific heat treatment conditions (Examples II-V) described in this document are applied, any film exhibiting excellent oxygen gas barrier property under high-humidity conditions can not be obtained from a mixture of PVA and poly(meth)acrylic acid.
Polyacrylic acid or partially neutralized products thereof are water-soluble polymers and widely used as water-absorbing materials, thickeners, flocculants, dispersants, treating agents for paper and fibers, and the like, making good use of their hydrophilic nature. The polyacrylic acid or partially neutralized products thereof may be formed into films from their solutions by a casting process. The resultant films are excellent in oxygen gas barrier property under dry conditions. However, these films are unsuitable for packaging of food containing a great amount of water because they show strong hydrophilic nature.